Multiple line and device levels in an integrated circuit are typically separated by insulating dielectric layers. Contact holes, more simply referred to as "contacts," are formed through the insulating layers to provide electrical contact between two or more conductive layers. The insulating layers must be grown or deposited to a minimum thickness. Too thin an insulating layer results in an intolerably high interlevel capacitance, which ties up otherwise available conduction carriers. Thus, as dimensions continue to be scaled down to the submicron level, the contact opening shrinks in size but the depth of the contact (through an insulating layer) must remain the same. In other words, the aspect ratio of contact openings increases as circuitry becomes more densely packed.
FIG. 1 illustrates a prior art conductive layer 3 deposited into a contact opening 4 to provide the metal-to-metal interlevel electrical contact. It should be understood that FIG. 1 is a schematic cross-sectional view, omitting the back wall for simplicity, but that the contact 4 is ordinarily etched in the shape of a cylinder. Unfortunately, physical vapor deposition of the conductive layer 3, such as metal evaporation or sputtering, produces poor step coverage into narrow contact openings having high-aspect ratios. During a metal sputter deposition, for example, metal builds quickly on a lip 5 of the contact 4.
Quick build-up of sputtered metal on the lip 5 of the contact 4, and the resultant poor step coverage, is produced in part and compounded by a shadow effect at a vertical sidewall 8. The faster deposition of metal on the contact lip 5 creates a cusp 9 into the mouth of the contact 4, sheltering the lower portions and thus even further slowing down deposition in the lower corner 6 of the contact 4. Eventually, the metal may pinch off at the contact mouth before significant metal is deposited in the lower corner 6. Where the contact has an aspect ratio of 1.0 or greater (contact height is equal to or greater than the diameter of the opening) the deposited metal is especially susceptible to pinching off and closing the contact. Voids or "keyholes" are thus created in filling the contact. As integrated circuits become more dense and the aspect ratios of metal-to-metal contacts continue to increase over 1.0 (1:1), the shadow effect becomes more pronounced.
FIG. 1A illustrates a contact made through an insulating layer 16 down to an active area 18 of a semiconductor substrate 20. Such contacts are typically even deeper and more narrow than metal-to-metal contacts such as that illustrated in FIG. 1. The insulating layer 16 (e.g., BPSG) is usually very thick in order to provide electrical isolation of underlying devices such as capacitors and transistors. Contacts to the substrate in dynamic random access memory (DRAM) cells are particularly deep, as the contact height is dictated by the height of adjacent cell capacitors. DRAM capacitors, in turn, are designed increasingly taller in order to provide adequate surface area for cell capacitance, despite shrinking available real estate.
At the same time, the active areas 18 to which contact must be made are constantly growing more narrow with each new generation of semiconductor chip, often confined between gate electrodes or word lines of a memory circuit, for example. Accordingly, metal-to-substrate contacts may range in aspect ratio from about 2:1 to 8:1.
Traditionally, contacts through thick insulating layers to silicon substrates have been filled with a conductive plug which is deposited by chemical vapor deposition ("CVD"). The most common material used for this purpose is tungsten (W), for which CVD methods are well-developed, resulting in conformal deposition and filling of the contact without keyholes. At the same time, use of tungsten plugs for direct contact with silicon substrates presents a number of problems. Most of these problems may be addressed by first depositing an initial conductive layer within the contact prior to depositing the tungsten. The material typically used for this layer is titanium, though other suitable materials are known.
The titanium serves several purposes. In the first place, the titanium at the surface of the silicon may be annealed to form a silicide (TiSi.sub.2 in its stoichiometric form) over the substrate surface. This significantly reduces the contact resistance between the active area and the tungsten plug through a better matching of energy levels. The process of forming the silicide (referred to as "silicidation"), further reduces contact resistance by breaking up native oxide (SiO.sub.2), an insulator which naturally forms at the silicon surface after the contact is opened. The layer of silicide also serves to prevent direct contact between the silicon substrate and tungsten, since tungsten tends to poison the substrate and interfere with electrical operations of transistors and other integrated devices. Furthermore, any titanium on the sidewalls of the contact improves adhesion, since CVD tungsten adheres poorly to the insulating layers, such as borophosphosilicate glass ("BPSG"), in which the contact is formed.
Unfortunately, titanium and many other appropriate metals are generally deposited by physical vapor deposition ("PVD") or sputtering. As with the metal-to-metal contact 4 illustrated in FIG. 1, contacts to the substrate 20 also tend to form keyholes or voids during the initial PVD.
FIG. 1A illustrates the complete blockage of the metal-to-substrate contact 14 caused by titanium (or other PVD metal) deposition. The active area 18 represents the bottom of the contact 14. In order to form a titanium substrate coating 22 of even 100 .ANG. at the bottom of the contact 14, a layer 24 often as thick as 1,300 .ANG. to 1,500 .ANG. must be deposited over the insulating layer 16. For higher aspect ratios of future generation circuits, even more titanium must be deposited for sufficient coverage of the bottom.
The small percentage of metal reaching the active area or substrate surface 18 is due to the natural tendency of PVD to more quickly grow at the corner of the contact hole than elsewhere, forming a metal cusp 26 which extends into the contact opening and eventually pinches it off. In the example illustrated in FIG. 1A the thick titanium layer 24 causes the titanium cusp 26 to pinch off, closing the mouth of the contact before enough titanium has reached the substrate. The substrate coating 22 is thus too thin. Furthermore, the contact 14 must be still be filled with further materials, such as the conventional tungsten plug, even though the contact 14 has been closed off.
FIGS. 2 and 3 illustrate that, even where the contact 14 is not completely pinched off during deposition of the titanium 22' and 24', the metal cusp 26' is problematic during later contact fill steps. FIG. 3 shows the result of a deposition of a CVD tungsten layer 30 and a silicidation step, forming a silicide layer over the substrate surface 18. Although CVD tungsten is generally much more conformal than the PVD titanium 24'. Nevertheless, even the CVD tungsten layer 30 cannot completely fill the contact 14 with such a large cusp 26' shadowing the contact 14, and certainly not if the contact 14 is completely closed off (FIG. 1A). A large void 32 thus forms within the plug, reducing the effective size of current flow path for the operational circuit. As aspect ratios of such contacts continually increase with each new generation of microchip, the risk of pinching off the contact opening increases in turn.
One method of improving step coverage of a PVD metal involves a process of sloping the sidewall of the contact, thus opening the contact in a tapered or cone shape. The minimum diameter of the bottom of the contact, however, is still limited by photolithographic resolution. Sloping the contact sidewall thus increases the total area occupied by the contact and reduces the allowable packing density. Such decreases in packing density are unacceptable in the face of current commercial requirements for the miniaturization of integrated circuits.
A need thus remains for an efficient method of filling high-aspect ratio contacts with conductive plugs for forming electrical contact with silicon substrates. Advantageously, such a method would also be compatible with conventional physical deposition techniques to allow use of a wide array of metals, including sputtered titanium.
According to one aspect of the invention, a method is provided for forming a conductive plug to contact a semiconductor substrate of an integrated circuit. The method comprises forming an insulating layer; forming a contact through the insulating layer to expose an active area of the substrate; depositing a first conductive layer over the insulating layer and into the contact; at least partially removing the cusp; and filling the contact with a CVD conductive layer.
According to another aspect of the invention, a method is provided for forming electrical contact between layers in an integrated circuit. The method comprises opening a contact through an insulating layer to expose a circuit element; depositing an initial conductive layer to form direct contact with a top surface of the insulating layer and the exposed circuit element; and removing a top portion of the initial conductive layer overlying the top surface of the insulating layer.
According to yet another aspect of the invention, a method is provided for forming electrical contact through an insulating layer to a semiconductor substrate. The method comprises etching a contact with an aspect ratio of greater than about 2:1 through the insulating layer to the substrate; depositing a metal layer over the insulating layer and into the contact, forming a metal cusp at a lip of the contact; at least partially removing the metal cusp; and depositing a second conductive layer into the contact.
According to still another aspect of the invention, a method is provided for forming a conductive plug to contact a semiconductor substrate of an integrated circuit. The method comprises forming a contact through an insulating layer, to expose an active area of the substrate; depositing a titanium layer over the insulating layer and into the contact; at least partially removing a titanium cusp shadowing the contact; annealing the titanium to form a silicide protective layer over the active area; and depositing a tungsten filler into the contact.
According to still another aspect of the invention, a method is provided for filling a contact through an insulating layer in an integrated circuit. The method comprises depositing an initial conductive layer into the contact and over the insulating layer; mechanically planarizing a top portion of the initial conductive layer; and filling the contact with a second conductive layer after planarizing the top portion.
According to still another aspect of the invention, a method is provided for forming a conductive contact in an integrated circuit. The method comprises forming a contact hole through an insulating layer; forming an initial conductive layer at the bottom of the contact and a top layer over the insulating layer; inverting the structure over an absorbent pad soaked with a viscous etchant to etch the top layer; and depositing a second conductive material into the contact hole after etching the top layer.
According to still another aspect of the invention, an integrated circuit with a contact plug in electrical contact with a semiconductor substrate is disclosed. The circuit comprises an insulating layer with a contact defined by a sidewall and a bottom surface, characterized by an aspect ratio greater than about 2:1; a first metallic layer at least partially lining the contact sidewall; and a conductive filler filling the contact; and a silicide layer interposed between the substrate and the conductive material.
Further aspects of the invention will be clear, in light of the disclosure herein, to one having skill in the art of integrated circuit fabrication.